Dual mode ion source for ion implantation

ABSTRACT

An ion source is disclosed for providing a range of ion beams consisting of either ionized clusters, such as B 2 H x   + , B 5 H x   + , B 10 H x   + , B 18 H x   + , P 4   +  or As 4   +  or monomer ions, such as Ge + , In + , Sb + , B + , As + , and P + , to enable cluster implants and monomer implants into silicon substrates for the purpose of manufacturing CMOS devices, and to do so with high productivity. The range of ion beams is generated by a universal ion source in accordance with the present invention which is configured to operate in two discrete modes: an electron impact mode, which efficiently produces ionized clusters, and an arc discharge mode, which efficiently produces monomer ions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of commonly owned co-pendingU.S. application Ser. No. 10/170,512, filed on Jun. 12, 2002, which wasnationalized from international patent application no. PCT/US00/33786,filed on Dec. 13, 2000, under 35 USC §371, which, in turn, claims thebenefit of U.S. provisional patent application No. 60/170,473, filed onDec. 13, 1999 and U.S. provisional application No. 60/250,080, filed onNov. 30, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ion source for the generation of ionbeams for doping wafers in the semiconductor manufacturing of PMOS andNMOS transistor structures to make integrated circuits and moreparticularly to a universal ion source configured to operate in dualmodes, for example, an arc discharge mode and an electron impact mode.

2. Description of the Prior Art

The Ion Implantation Process

The fabrication of semiconductor devices involves, in part, theformation of transistor structures within a silicon substrate by ionimplantation. The ion implantation equipment includes an ion sourcewhich creates a stream of ions containing a desired dopant species, abeam line which accelerates and focuses the ion stream into an ion beamhaving a well-defined energy or velocity, an ion filtration system whichselects the ion of interest, since there may be different species ofions present within the ion beam, and a process chamber which houses thesilicon substrate upon which the ion beam impinges ; the ion beampenetrating a well-defined distance into the substrate. Transistorstructures are created by passing the ion beam through a mask formeddirectly on the substrate surface, the mask being configured so thatonly discrete portions of the substrate are exposed to the ion beam.Where dopant ions penetrate into the silicon substrate, the substrate'selectrical characteristics are locally modified, creating source, drainand gate structures by the introduction of electrical carriers: such as,holes by p-type dopants, such as boron or indium, and electrons byn-type dopants, such as phosphorus or arsenic, for example.

Prior Art Ion Sources

Traditionally, Bernas-type ion sources have been used in ionimplantation equipment. Such ion sources are known to break downdopant-bearing feed gases,such as BF₃, AsH₃ or PH₃, for example, intotheir atomic or monomer constituents, producing the following ions incopious amounts: B⁺, As⁺ and P⁺. Such ion sources are known to produceextracted ion currents of up to 50 mA, enabling up to 20 mA of filteredion beam at the silicon substrate. Bernas type ion sources are known ashot plasma or arc discharge type sources and typically incorporate anelectron emitter, either a naked filament cathode or anindirectly-heated cathode, and an electron repeller, or anticathode,mounted opposed to one another in a so-called “reflex” geometry. Thistype of source generates a plasma that is confined by a magnetic field.

Recently, cluster implantation sources have been introduced into theequipment market. These ion sources are unlike the Bernas-style sourcesin that they have been designed to produce “clusters”, or conglomeratesof dopant atoms in molecular form, e.g., ions of the form AS_(n) ⁺,P_(n) ⁺, or B_(n)H_(m) ⁺, where n and m are integers, and 2≦n≦18. Suchionized clusters can be implanted much closer to the surface of thesilicon substrate and at higher doses relative to their monomer (n=1)counterparts, and are therefore of great interest for formingultra-shallow p-n transistor junctions, for example in transistordevices with gate lengths of 65 nm, 45 nm, or 32 nm. These clustersources preserve the parent molecules of the feed gases and vaporsintroduced into the ion source. The most successful of these have usedelectron-impact ionization, and do not produce dense plasmas, but rathergenerate low ion densities at least 100 times smaller than produced byconventional Bernas sources.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to an ion source for providing arange of ion beams consisting of either ionized clusters such as B₂H_(x)⁺, B₅H_(x) ⁺, B₁₀H_(x) ⁺, B₁₈H_(x) ⁺, P₄ ⁺ or As₄ ⁺ or monomer ions,such as Ge⁺, In⁺, Sb⁺, B⁺, As⁺, and P⁺, to enable cluster implants andmonomer implants into silicon substrates for the purpose ofmanufacturing CMOS devices, and to do so with high productivity. This isaccomplished by the novel design of an ion source which can operate intwo discrete modes: electron impact mode, which efficiently producesionized clusters, or arc discharge mode, which efficiently producesmonomer ions.

Borohydride molecular ions are created by introducing gaseous B₂H₆,B₅H₉, B₁₀H₁₄, or B₁₈H₂₂ into the ion source where they are ionizedthrough a “soft” ionization process, such as electron impact ionization,which preserves the number of boron atoms in the parent molecule (thenumber of hydrogens left attached to the ion may be different from thatof the parent). Likewise, As vapor or P vapor can be introduced into theion source (from a vaporizer which sublimates elemental As or P) toproduce an abundance of As₄ ⁺, As₂ ⁺, and As⁺, or P₄ ⁺, P₂ ⁺, and P⁺ions. The mechanism for producing As and P clusters from elemental vaporwill be described in more detail below. Monomer ions are produced bycreating an arc discharge within the ion source, producing a denseplasma and breaking down the feed gases BF₃, AsH₃, PH₃, SbF₅, InCl₃,InF₃ and GeF₄ into their constituent atoms. This provides high currentsof Ge⁺, In⁺, Sb⁺, B⁺, As⁺, and P⁺ ions as required by many semiconductorprocesses today. The invention, as described in detail below, isdisclosed by novel methods of constructing and operating a single oruniversal ion source which produces these very different ion species,i.e., both clusters and monomers, and switching between its two modes ofoperation quickly and easily, enabling its efficient use insemiconductor manufacturing.

Production of Clusters of Arsenic and Phosphorus

An object of this invention is to provide a method of manufacturing asemiconductor device, this method being capable of forming ultra-shallowimpurity-doped regions of N-type conductivity in a semiconductorsubstrate by implanting ionized clusters of the form P₄ ⁺ and As₄ ⁺.

A further object of this invention is to provide for an ion implantationsource and system for manufacturing semiconductor devices, which hasbeen designed to form ultra shallow impurity-doped regions ofN-conductivity type in a semiconductor substrate through the use ofcluster ions of the form P₄ ⁺ and As₄ ⁺.

According to one aspect of this invention, there is provided a method ofimplanting cluster ions comprising the steps of: providing a supply ofmolecules each of which contains a plurality of either As or P dopantatoms into an ionization volume, ionizing the molecules into dopantcluster ions, extracting and accelerating the dopant cluster ions withan electric field, selecting the desired cluster ions by mass analysis,and implanting the dopant cluster ions into a semiconductor substrate.

Economic Benefits of As and P Clusters

While the implantation of P-type clusters of boron hydrides forsemiconductor manufacturing has been demonstrated, no N-type cluster hasbeen documented which produces large ionized clusters in copiousamounts. If ions of the form P_(n) ⁺ and As_(n) ⁺ with n=4 (or greater)could be produced in currents of at least 1 mA, then ultra-low energy,high dose implants of both N- and P-type conductivity would be enabled.Since both conductivity types are required by CMOS processing, such adiscovery would enable clusters to be used for all low energy, high doseimplants, resulting in a dramatic increase in productivity, with aconcomitant reduction in cost. Not only would cost per wafer declinedramatically, but fewer ion implanters would be required to processthem, saving floor space and capital investment.

Process Benefits of As and P Clusters

The preferred method of forming drain extensions for sub-65 nm devicesis expected to incorporate a wafer tilt ≧30 deg from the substratenormal, in order to produce enough “under the gate” dopantconcentration, without relying on excessive dopant diffusion broughtabout by aggressive thermal activation techniques. Excellent beamangular definition and low beam angular divergence are also desired forthese implants; while high current implanters tend to have large angularacceptances and significant beam non-uniformities, medium currentimplanters meet these high-tilt and precise angle control requirements.Since medium-current implanters do not deliver high enough currents,their throughput on high-dose implants is too low for production. If ionimplanters could produce the required low-energy beams at high doserates, great economic advantage would be achieved. Since drainextensions are the shallowest of implants, they are also at the lowestenergies (about 3 keV As at the 65 nm node, for example); the long,complicated beamlines which typify medium-current implanters cannotproduce enough current at low energy to be useful in manufacturing suchdevices. The use of As₄ ⁺ and P₄ ⁺ cluster implantation inmedium-current beam lines and other scanned, single-wafer implantersextends the useful process range of these implanters to low energy andto high dose. By using high currents of these clusters, up to a factorof 16 in throughput increase can be realized for low-energy, high dose(≧10¹⁴/cm²) implants with effective As and P implant energies as low as1 keV per atom.

The Chemical Nature of Arsenic and Phosphorus

As is generally known, elemental, solid As and P are known to exist in atetrahedral form(i.e., as white phosphorus, P₄, and as yellow arsenic,As₎. They would therefore seem to be ideal candidates for producingtetramer ions in an ion source. However, while these compounds can besynthesized, they are more reactive, and hence more unstable, than theirmore common forms, i.e., red P and grey As metals. These latter formsare easily manufactured, stable in air, and inexpensive. Importantly, itturns out that when common red P and grey As are vaporized, theynaturally form primarily P₄ and As₄ clusters in the vapor phase! [see,for example, M. Shen and H. F. Schaefer III, J. Chem. Phys. 101 (3) pp.2261-2266, 1 Aug. 1994.; Chemistry of the Elements, 2^(nd) Ed., N. NGreenwood and A. Earnshaw, Eds., Butterworth-Heiemann Publishers,Oxford, England, 2001, Chap. 13, p. 55; R. E. Honig and D. A. Kramer,RCA Review 30, p. 285, June 1969.] Electron diffraction studies haveconfirmed that in the vapor phase the tetrahedral As₄ predominates. Thistetrahedral phase is delicate, however, and is readily dissociated, forexample, by exposure to ultraviolet light or x-rays, and dissociates inplasmas of the type formed by conventional ion sources. Indeed, it isknown that As₄ quite readily dissociates into 2 As₂ under energeticlight bombardment.

Significant currents of ionized As₄ and P₄ clusters can be produced byvaporizing solid forms of As and P (either the amorphous or tetrahedralsolid phases) and preserving these clusters through ionization in anovel electron-impact ionization source, demonstrating that the clusterssurvive electron impact.

Although prior art ion sources have used vaporized solid As and P togenerate ion beams, the tetramers have not been preserved. The ionsproduced by these arc discharge sources have consisted of principallymonomers and dimers. Since the tetramer forms As₄ and P₄ are delicateand easily dissociated by the introduction of energy, to preserve them,the source should be free from excessive UV (such as emitted by hotfilaments, for example) and most importantly, be ionized by a “soft”ionization technique, such as electron impact. As will be discussed inmore detail below, this technique is useful in creating As₄ ⁺ ions fromvaporized elemental arsenic and phosphorus.

Advantages of the Novel Ion Source for As₄ and P₄ Production

The ion source of the present invention introduces gaseous As₄ and P₄vapors through a vaporizer which heats solid feed materials, such aselemental As or P, and conducts the vapor through a vapor conduit intothe ionization chamber of the ion source. Once introduced into theionization chamber of the ion source, the vapor or gas interacts with anelectron beam which passes into the ionization volume from an externalelectron gun, forming ions. The vapor is not exposed to a hot,UV-producing cathode since the electron gun is external to theionization volume and has no line-of-sight to the vapors. The ions arethen extracted from a rectangular aperture in the front of theionization volume by electrostatic optics, forming an ion beam.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1 is a schematic diagram of an exemplary ion beam generation systemin accordance with the present invention.

FIG. 2 is a schematic diagram of an alternative embodiment of theexemplary ion beam generation system illustrated in FIG. 1,illustratinga solid vapor source and an in-situ cleaning system.

FIG. 3 a is a schematic representation of the basic components of theion source in accordance with the present invention which includes anelectron gun, an indirectly-heated cathode, a source liner, a cathodeblock, a base, an extraction aperture, a source block, and a mountingflange.

FIG. 3 b is an exploded view of the ion source of the present invention,illustrating the major subsystems of the ion source

FIG. 4 a is an exploded isometric view of the ion source illustrated inFIG. 3 a, shown with the mounting flange assembly, electron gunassembly, indirectly heated cathode assembly and the extraction apertureplate removed.

FIG. 4 b is an exploded isometric view of the ionization volume linerand the interface or base block showing the plenum and the plenum portsin the interface block.

FIG. 4 c is an isometric view of the ionization volume assembly in whichthe ionization volume is formed from a cathode block, an interfaceblock, and a magnetic yoke assembly, shown with the ionization volumeliner removed.

FIG. 5 a is an exploded isometric view of a indirectly-heated cathode(IHC) assembly in accordance with one aspect of the present invention.

FIG. 5 b is an enlarged exploded view of a portion of the IHC assembly,illustrating the IHC, a filament, a cathode sleeve, and a portion of acathode plate.

FIG. 5 c is an elevational view in cross section of the IHC assemblyillustrated in FIG. 5 b.

FIG. 5 d is an isometric view of a water-cooled cathode block shownassembled to the IHC assembly illustrated in FIG. 5 a in accordance withone aspect of the invention.

FIG. 5 e is an elevational view of the assembly illustrated in FIG. 5 dillustrating the cathode block and the cathode plate of the IHC assemblyin section.

FIG. 5 f is an isometric view of a magnetic yoke assembly whichsurrounds the cathode block and ionization volume In accordance with thepresent invention.

FIG. 6 a is an isometric view of an emitter assembly which forms aportion of the external electron gun assembly in accordance with oneaspect of the present invention.

FIG. 6 b is an isometric view of an electron gun assembly in accordancewith the present invention shown with an electrostatic shield assemblyremoved.

FIG. 7 is an isometric view illustrating a magnetic circuit associatedwith the electron gun and ionization volume yoke assembly.

FIG. 8 is an isometric view of an exemplary dual hot vaporizer assemblyin accordance with one aspect of the present invention.

FIG. 9 a is an isometric view of a source block in accordance with thepresent invention

FIG. 9 b is similar to FIG. 9 a but shown with the hot vaporizerassembly removed.

FIG. 10 is a diagram which illustrates the typical voltages applied toeach element of the ion source when operating in electron-impactionization mode.

FIG. 11 is similar to FIG. 10 but indicates the typical voltages appliedto each element of the ion source when operating in arc discharge mode.

FIGS. 12 a and 12 b are logic flow diagrams illustrating the sequence ofsteps required to establish each operating mode in succession.

FIG. 13 is a diagram which shows the thermal interfaces between sourceblock, interface block, cathode block, and the ionization volume liner.

FIG. 14 is a side view in cross section, of the source assembly, cut inthe y-z plane.

FIG. 15 is similar to FIG. 14 but cut in the x-y plane.

FIG. 16 is similar to FIG. 14 but cut in the x-z plane

FIG. 17 is a photograph of the source with the front aperture plateremoved, showing the indirectly heated cathode and the ionization volumeliner.

FIG. 18 is a photograph showing the mounting flange with feedthroughs,shown with the vaporizers removed.

FIG. 19 is a plot of mass-analyzed B₁₈H_(x) ⁺ beam current delivered toan implanter Faraday cup positioned 2 meters from the ion source anddownstream from an analyzer magnet on the left vertical axis, and totalion current extracted from the same ion source shown on the rightvertical axis, as a function of vapor flow into the ion source.

FIG. 20 is a B₁₈H₂₂ mass spectrum collected from the ion source of thepresent invention.

FIG. 21 is a PH₃ mass spectrum collected from the ion source of thepresent invention.

FIG. 22 is an AsH₃ mass spectrum collected from the ion source of thepresent invention.

FIG. 23 is a P spectrum showing the monomer P⁺, the dimer P₂ ⁺, thetrimer P₃ ⁺, and the tetramer P₄ ⁺.

FIG. 24 is a As spectrum showing the monomer As⁺, the dimer As₂ ⁺, thetrimer As₃ ⁺, and the tetramer As₄ ⁺.

DETAILED DESCRIPTION

The present invention relates to ion source for providing a range of ionbeams consisting of either ionized clusters,such as B₂H_(x) ⁺, B₅H_(x)⁺, B₁₀H_(x) ⁺, B₁₈H_(x) ⁺, P₄ ⁺ or As₄ ⁺ or monomer ions,such as Ge⁺,In⁺, Sb⁺, B⁺, As⁺, and P⁺, to enable cluster implants and monomerimplants into silicon substrates for the purpose of manufacturing CMOSdevices, and to do so with high productivity. The range of ion beams isgenerated by a universal ion source in accordance with the presentinvention which is configured to operate in two discrete modes: anelectron impact mode, which efficiently produces ionized clusters, andan arc discharge mode, which efficiently produces monomer ions.

The universal ion source in accordance with the present invention isillustrated and described below. FIG. 14 shows, in cross section, a cutin the y-z plane (i.e., side view) through the ion source assembly inaccordance with the present invention. FIG. 15 is similar to FIG. 14,but shows, in cross section, a cut in the x-y plane through the sourceassembly. FIG. 16 shows, in cross section, a cut in the x-z planethrough the source assembly. FIG. 17 is a photograph of the source withthe front aperture plate removed, showing the indirectly heated cathodeand the ionization chamber liner. FIG. 18 is a photograph showing themounting flange with feed throughs with the vaporizers removed.

In order to efficiently produce ionized clusters, the ion source of thepresent invention incorporates the following features:

-   -   An electron-impact ionization source is provided, for example an        electron gun which is located external to the ionization volume        and out of line-of-sight of any process gas or vapors exiting        the ionization volume, and the vapors in the ionization chamber        are likewise not exposed to electromagnetic radiation emitted by        the hot cathode in the electron gun;    -   When operating in the electron-impact mode, the surfaces exposed        to the vapor introduced into the source are held within a        temperature range which is low enough to prevent dissociation of        the temperature-sensitive parent molecule, and high enough to        prevent or limit unwanted condensation of the vapors onto said        surfaces;    -   Multiple vaporizers are provided which can produce a stable flow        of vapor into the source (the vaporization temperatures of the        solid borohydride materials B₁₀H₁₄ and B₁₈H₂₂ range from 20 C to        120 C, while solid elemental materials, such as As and P,        require heating in the range between 400 C and 550 C to provide        the required vapor flows. Thus, one or more “cold” vaporizers        and one or more “hot” vaporizers are incorporated into the ion        source.

In order to efficiently produce monomer ions, the ion source of thepresent invention also incorporates the following features:

-   -   An electron source (cathode), a repeller (anticathode) and a        magnetic field are incorporated into the ion source in a        “reflex” geometry, wherein a strong magnetic field is oriented        substantially parallel to the ion extraction aperture of the ion        source, along a line joining the electron source and repeller;    -   Electronics are provided so that an arc discharge can be        sustained between the cathode and the anticathode, such that a        plasma column is sustained along the magnetic field direction,        i.e., parallel and in proximity to the ion extraction aperture;    -   An ionization volume liner (an “inner chamber”) is provided        within the ion source, enclosing the ionization volume, and is        allowed to reach a temperature well in excess of 200 C during        arc discharge operation in order to limit condensation of As, P        and other species onto the walls surrounding the ionization        volume;    -   A process gas feed is provided to supply conventional gaseous        dopant sources into the ion source.

Other novel features are provided in the ion source to enablereliability and performance: It is a feature of the invention that theion source incorporates an in-situ chemical cleaning process, preferablyby the controlled introduction of atomic fluorine gas, and the materialsused to construct the elements of the ion source are selected frommaterials resistant to attack by F:

The ionization chamber liner may be fabricated from titanium diboride(TiB₂), which is resistant to attack by halogen gases, and possessesgood thermal and electrical conductivity, but may also be usefullyfabricated of aluminum, graphite or other electrical and thermalconductor which is not readily attacked by flourine;

The arc discharge electron source may be an indirectly-heated cathode,and the portion of which exposed to the cleaning gas may be formed athick tungsten, tantalum or molybdenum disk, and is therefore much morerobust against failure in a halogen environment than a naked filament;

The indirectly-heated cathode assembly is mechanically mounted onto awater-cooled aluminum “cathode block” so that the, limiting itsradiative heat load to the ionization chamber and liner (we note thataluminum passivates in a F environment, and is therefore resistant tochemical etch); this enables rapid cool down of the cathode between thetime it is de-energized and the onset of an in-situ cleaning cycle,reducing the degree of chemical attack of the refractory metal cathode

The electron gun which is energized during electron-impact ionization(i.e., during cluster beam formation) is remote from the ionizationvolume, mounted externally and has no line-of-sight to the F gas loadduring an in-situ clean, and therefore is robust against damage by Fetching.

Other novel features are incorporated to improve source performance andreliability:

-   -   The aluminum cathode block or frame is at cathode potential,        eliminating the risk of cathode voltage shorts which are known        to occur between indirectly-heated cathodes and the source        chambers of prior art sources. This block also conveniently        forms the repeller structure, being at cathode potential,        thereby obviating the need for a dedicated electron repeller or        anticathode;    -   The ionization volume liner is surrounded by a cathode block and        a base; the aluminum base and cathode block are held in thermal        contact with a temperature-controlled source block through        thermally conductive, but electrically insulating elastomeric        gaskets. This feature limits the maximum temperature of the        block and base to near the source block temperature (the source        block is typically held below 200 C);    -   The ionization volume liner is in thermal contact with the base        through a high-temperature, thermally and electrically        conductive gasket, such as aluminum, to limit its maximum        temperature excursion while insuring its temperature is higher        than that of the cathode block and base; Unlike other known ion        sources, no ionization chamber per se is provided.    -   The source magnetic field is provided by a magnetic yoke        assembly which surrounds the ionization chamber assembly. It is        embedded in the cathode block. This provides a means for keeping        the yoke assembly at a temperature well below the Curie        temperature of its permanent magnets.    -   The ion source operates in two discrete modes: electron impact        mode and arc discharge mode. The operating conditions for each        are quite different as described in detail below.

When operating in electron impact mode, the following conditions aremet:

-   -   The source block is held at a temperature between about 100° C.        and 200° C. Depending on which specie is run in the ion source;        this provides a reference temperature for the source, preventing        condensation of the source material, such as borohydride or        other source materials;    -   The indirectly heated cathode is not energized, and cooling        water is not run in the cathode block. The cathode block comes        to thermal equilibrium with the base, with which it is in        thermal contact through a thermally conductive, but electrically        insulating, gasket (the base is in turn in good thermal contact        with the source block, and so rests near the source block        temperature);    -   The cathode block is held at the same potential as the base and        the ionization volume liner;    -   The electron gun is energized by applying a negative potential        to the electron emitter (i.e. the cathode), and applying a        positive potential to the anode and the gun base (i.e. the        potential of the local environment of the electron beam as it        propagates through the gun). The cathode and anode voltages are        measured with respect to the ionization volume. This enables a        “deceleration” field to act on the electron beam as it        propagates between the gun base and ionization volume so that        the energy of the electrons which ionize the gas or vapor can be        varied independently of the energy of the electron beam        propagating within the gun, and in particular be reduced to        effect more efficient ionization of the gas molecules;    -   A permanent magnetic field provides confinement of the electron        beam as it enters and transits the ionization chamber, enabling        a uniform ion density to be created adjacent to, and along the        ion extraction aperture of the ion source;    -   The TiB₂ liner (which can also be made of SiC, B₄C, Al, C, or        any other suitable electrically conductive material which is not        a deleterious contaminant in silicon circuits) is in thermal        contact with the base (which is in thermal continuity with the        source block) through an electrically and thermally conductive        high-temperature gasket, and so will settle close to the source        block temperature, since very little power (typically <10 watts)        is dissipated by the electron beam within the ionization volume        The liner is thus always at the same potential as the ionization        volume and the source block.

When operating in arc discharge mode, the following conditions are met:

-   -   The source block is held at between 100° C. and 200° C.    -   The indirectly heated cathode is energized, and cooling water is        run in the cathode block. The cathode block temperature is thus        maintained near to the water temperature, and cooler than the        base, which is in thermal contact with the source block;    -   The cathode block is held at the same potential as the cathode,        up to 100V negative with respect to the liner which surrounds        the ionization volume. Since the cathode block also comprises        the repeller or anticathode, it is also at cathode potential. In        the presence of the permanent axial magnetic field, this enables        a true “reflex” geometry and hence a stable plasma column. The        arc current is absorbed by the liner, whose potential        establishes the plasma potential.    -   The electron gun is not energized, the electron emitter is set        to source block potential, and the gun base is set to cathode        block potential. This prevents any net field from penetrating        from the gun base through the electron entrance aperture in the        cathode block.    -   With the indirectly-heated cathode energized and an arc        discharge initiated, the liner is exposed to a significant        radiative heat load. This allows the liner to reach an        equilibrium temperature well in excess of the base. The maximum        temperature differential can be “tuned” by reducing or        increasing the thermal contact between liner and base.

Referring to FIG. 1, a schematic diagram of an exemplary ion beamgeneration system which incorporates an ion source in accordance withthe present invention is illustrated. As shown in this example, the ionsource 400 is adapted to produce an ion beam for transport to an ionimplantation chamber for implant into semiconductor wafers or flat-paneldisplays. The ion beam generation system includes an ion source 400, anextraction electrode 405, a vacuum housing 410, a voltage isolationbushing 415 of electrically insulative material, a vacuum pumping system420, a vacuum housing isolation valve 425, a reactive gas inlet 430, afeed gas and vapor inlet 441, a vapor source 445, a feed gas source 450,a reactive gas source 455, an ion source high voltage power supply 460and an ion beam transport housing 411. The ion source 400 produces aresultant ion beam illustrated by the arrow 475.

The ion source 400 is constructed to provide cluster ions and molecularions, for example the borohydride ions B₁₀H_(x) ⁺, B₁₀H_(x) ⁻, B₁₈H_(x)⁺, and B₁₈H_(x) ⁻ or, and alternatively, more conventional ion beams,such as P⁺, As⁺, B⁺, In⁺, Sb⁺, Si⁺, and Ge. The gas and vapor inlet 441for gaseous feed material to be ionized is connected to a suitable vaporsource 445, which may be in close proximity to gas and vapor inlet 441or may be located in a more remote location, such as in a gasdistribution box, located elsewhere within a terminal enclosure.

A terminal enclosure is a metal box, not shown, which encloses the ionbeam generating system. It contains required facilities for the ionsource, such as pumping systems, power distribution, gas distribution,and controls. When mass analysis is employed for selection of an ionspecies in the beam, the mass analyzing system may also be located inthe terminal enclosure.

In order to extract ions of a well-defined energy, the ion source 400 isheld at a high positive voltage (in the more common case where apositively-charged ion beam is generated) with respect to an extractionelectrode assembly 405 and a vacuum housing 410 by a high voltage powersupply 460. The extraction electrode assembly 405 is disposed close toand aligned with an extraction aperture 504 on an extraction apertureplate which forms a portion of the ionization volume 500. The extractionelectrode assembly consists of at least two aperture-containingelectrode plates, a so-called suppression electrode 406 closest to theionization volume 500, and a “ground” electrode 407. The suppressionelectrode 406 is biased negative with respect to a ground electrode 407to reject or suppress unwanted electrons which are attracted to thepositively-biased ion source 400 when generating positively-charged ionbeams. The ground electrode 407, vacuum housing 410, and terminalenclosure (not shown) are all at the so-called terminal potential, whichis at earth ground unless it is desirable to float the entire terminalabove ground, as is the case for certain implantation systems, forexample for medium-current ion implanters. The extraction electrode 405may be of the novel temperature-controlled metallic design, describedbelow.

In accordance with another aspect of the invention, the ion source 400,illustrated in of FIG. 1, may be configured for in situ cleaning, i.e.without the ion source being removed from its operating position in thevacuum housing, and with little interruption of service. Indeed, for ionsources suitable for use with ion implantation systems, e.g. for dopingsemiconductor wafers, the source chamber or ionization volume 500 issmall, having a volume, for example, less than about 100 ml, and aninternal surface area of, for example, less than about 200 cm², and isconstructed to receive a flow of the reactive gas, e.g. atomic fluorineor a reactive fluorine-containing compound at a flow rate of less thanabout 200 Standard Liters Per Minute. As such, a dedicated endpointdetector 470, in communication with the vacuum housing 410 may be usedto monitor the reactive gas products during chemical cleaning.

FIG. 2 illustrates an embodiment of an ion source, similar to FIG. 1,that is configured for conducting in-situ chemical cleaning of the ionsource 400 including the extraction electrode assembly 405. The in situcleaning system is described in detail in International PatentApplication No. PCT/US2004/041525, filed on Dec. 9, 2004, herebyincorporated by reference. Briefly, three inlet passages are integratedinto ion source 400, respectively. One inlet passage is for reactive gas430 from a plasma source 455. Another inlet passage is for feed gas 435from one of a number of storage volumes 450 selected. The third inlet isfor feed vapor 440 from a vaporizer 445. The plasma-based reactive gassource 455 is biased at the high voltage of the ion source 400. Thisenables the remote plasma source 455 to share control points of the ionsource 400 and also enables the cleaning feed gas 465 and argon purgegas from storage source 466 to be supplied from an ion source gasdistribution box, which is at source potential. Also shown is adifferent type of endpoint detector, namely a Fourier Transform Infrared(FTIR) optical spectrometer. This detector can function ex-situ (outsideof the vacuum housing), through a quartz window. Instead, as shown inFIG. 2, an extractive type of FTIR spectrometer may be used, whichdirectly samples the gas in the vacuum housing 410 during cleaning. Alsoa temperature sensor TD may sense the temperature of the de-energizedionization chamber by sensing a thermally isolated, representativeregion of the surface of the chamber. The sensor TD can monitor heatproduced by the exothermic reaction of F with the contaminating deposit,to serve as end-point detection.

FIG. 3 a is a simplified schematic representation of the basiccomponents of the ion source, indicating the electron gun cathode 10,the indirectly-heated cathode (IHC) 20, an ionization volume liner 30, acathode block 40, a base or interface block 50, extraction apertureplate 60, a source block 70, and a mounting flange 80. The ionizationvolume liner 30 is preferably made of TiB₂ or aluminum, but may beusefully constructed of SiC, B₄C, C, or any other suitable electricallyconductive material which is not a deleterious contaminant in siliconcircuits, and can sustain an operating temperature of between 100 C and500 C. The cathode block 40 is preferably of aluminum due to its highthermal and electrical conductivity, and resistance to attack by halogengases. Al also allows for direct water cooling since it is non-porousand non-hydroscopic. Other materials may be used such as refractorymetals like tungsten and molybdenum which have good electrical andthermal properties; however they are readily attacked by halogen gases.Another consideration for the cathode block is compatibility with ionbombardment of P⁺, As⁺, and other species produced under arc dischargeoperation. Since the cathode block is unipotential with the IHC cathode20, it is subject to erosion by ion bombardment of plasma ions. Thesputter rates of materials under bombardment by ions of interesttherefore must be considered as it will impact useful source life. Thebase 50, again, is preferably made of aluminum, but can be made ofmolybdenum or other electrically and thermally conductive materials.Since the source block 70, mounting flange 80, and ion extractionaperture 60 are typically operated at 200 C or below, they can beusefully constructed of aluminum as well The ionization volume liner 30surrounds an ionization volume 35 and is in light thermal contact withthe mounting base 50, which is itself in good thermal contact with thesource block 70. Except for a slot through the ionization volume liner30 and the extraction aperture plate 60 through which ions pass, theionization volume of the ion source is fully bounded by a cylindricalbore through the ionization volume liner 30 and the top and bottomplates of the cathode block 40. The source block 70 is temperaturecontrolled to up to 200 C, for example. Thus, when the electron gun 10is active, very little power is transferred to the ionization volumeliner 30, the temperature of which is close to that of the source block70. When the IHC 100 is energized, the ionization volume liner 30 isexposed to hundreds of watts of power and can attain a much highertemperature than the source block 70 (up to 400 C or higher), which isbeneficial to limit condensation of gases onto the surface of theionization volume liner 30.

FIG. 3 b is an exploded isometric view of the ion source of the presentinvention, showing its major subsystems. The ion source includes an ionextraction aperture plate 60, an ionization volume or chamber assembly90, an IHC assembly 100, an electron gun assembly 110, a source blockassembly 120, and a mounting flange assembly 130. The ion source alsoincludes a low-temperature vaporizer (not shown) coupled to a port 135.A vapor conduit 137 is used to transport the vapor into the ionizationassembly 90. The ion source also includes dual hot vapor inlet ports138, a process gas inlet port 139, and an optional reactive gas inletport 140. In an exemplary, embodiment atomic F may fed to the ionizationvolume assembly 90 via the reactive gas inlet port 140. Vaporized As, P,or SbO₃ into the dual hot vapor inlet ports 138 while B₁₈H₂₂ vapor maybe applied to the vapor conduit 137.

FIG. 4 a is an exploded isometric view of the ion source in accordancewith the present invention, shown with the mounting flange assembly 130,electron gun assembly 110, indirectly heated cathode assembly 100 andthe extraction aperture plate 60 removed. The ion source includes asource block 120, a cathode block 40, mounting base or interface block50, an ionization volume or source liner 30, a liner gasket 115, a basegasket 125, and a cathode block gasket 127. As will be discussed in moredetail below and as illustrated in FIG. 4 c, when the magnetic yokeassembly 150 is added, these parts form an ionization volume assembly 90(FIG. 3 b). The gaskets 125 and 127 are electrically insulating,thermally conductive gaskets, fabricated from polymer compounds, forexample. Their purpose is to prevent thermal isolation of the partswhile allowing for potential differences between the mating parts. Forexample, the cathode block 40 is at several hundred volts below the baseor interface block 50 potential during arc discharge operation, and somust be electrically isolated. However, during electron impactoperation, the cathode block 40 should be near the temperature of thebase or interface block 50, and so it cannot be thermally isolated. Thegasket 115, however, is a metal gasket which forms the interface betweenthe ionization volume liner 30 and the base or interface block 50. Metalwas chosen because of its ability to withstand the higher temperaturesthe ionization volume liner 30 will reach during arc dischargeoperation. Since the base or interface block 50 is effectively heat sunkto the source block 120 (which is a constant temperature reservoir,i.e., it is actively temperature controlled through embedded ohmicheaters coupled to a closed-loop controller), it tracks near the sourceblock 70 temperature. The source block 70 is actively temperaturecontrolled, and the separate source elements track this temperaturethrough carefully selected thermal contact paths, as described in FIG.13. Closed loop control of the source block 70 temperature may beimplemented using a conventional PID controller, such as the Omron E5CKdigital controller, which can be used to control the duty cycle of thepower delivered to the ohmic heaters embedded in the source block, as isknown in the art.

FIG. 4 b is an exploded isometric view of the ionization volume liner 30and the interface or base block 50, showing the plenum and the plenumports in the interface block 50. The several gas and vapor inlet ports,namely vapor port 137, reactive gas port 140, process gas port 139, anddual hot vapor ports 141 a and 141 b, feed into a gas plenum 45, formedin the base or interface block 50. The interface block 50 is providedwith one or more through holes 142 a and 142 b to accommodate mountingconventional fasteners (not shown) to secure the interface block 50 tothe source block 120 and thereby establish electrical conductivitybetween the interface block 50 and the source block 120). The gas plenum45 may be cavity machined into the interface block 50 and is used tocollect any of the gases fed into the plenum 45 and feed them intomultiple liner ports 32. The multiple liner ports 32 are configured in a“shower head” design to distribute the gases along different directionsinto the ionization volume 35 within the ionization volume liner 30. Bytransporting all of the gases or vapors into the plenum 45, which actsas a ballast volume, which then feeds the gases through a shower headdirectly into the ionization volume 35, produces a uniform distributionof gas or vapor molecules within the ionization volume 35. Such aconfiguration results in a more uniform distribution of ions presentedto extraction aperture 60, and the subsequent formation of a morespatially uniform ion beam.

FIG. 4 c is an isometric view of the ionization volume assembly 90,shown with the ionization volume liner removed. The ionization volumeassembly 90 is formed from the cathode block 40, the interface block 50,and the magnetic yoke assembly 150. The magnetic yoke assembly 150 isconstructed of magnetic steel and conducts the magnetic flux produced bya pair of permanent magnets 151 a and 151 b around through ionizationvolume assembly 90, producing a uniform magnetic field of about 120Gauss, for example, within the ionization volume 35. During electronimpact operation, this permanent field confines the electron beam sothat the ions are produced in a well-defined, narrow column adjacent tothe ion extraction aperture 60. During arc discharge mode, the samefield provides confinement for the plasma column between cathode and theupper plate of the cathode block 40, which serves as an anticathode.

FIG. 5 a is an exploded view of the indirectly-heated cathode (IHC)assembly 100. IHC assemblies are generally known in the art. Examples ofsuch IHC assemblies are disclosed in U.S. Pat. Nos. 5,497,006;5,703,372; and 6,777,686, as well as US Patent Application PublicationNo. US 2003/0197129 A1, all hereby incorporated by reference. Theprinciples of the present able invention are applicable to all such IHCassemblies. An alternate IHC assembly 100 in for use with the presentinvention includes an indirectly-heated cathode 160, a cathode sleeve161, a filament 162, a cathode plate 163,a pair of filament clamps 164 aand 164 b, a pair of filament leads 165 a and 165 b, and a pair ofinsulators 167 a and 167 b (not shown). The filament 162 emits up to 2A, for example, of electron current which heats the indirectly-heatedcathode 160 to incandescence by electron bombardment. Since the filament162 is held at a negative potential of up to 1 kV below the cathodepotential, up to 2 kW of electron beam heating capacity is available forcathode heating, for example. In practice, heating powers of between 1kW and 1.5 kW are sufficient, although for very high arc currents (inexcess of 2 A of arc) higher power can be required. The cathode 160 isunipotential with the cathode mounting plate 163. The insulators 167 aand 167 b are required to stand off the filament voltage of up to 1 kV.

Referring now to FIGS. 5 b and 5 c, the IHC 160 is located onto thecathode plate 163 via a flange 159 and is locked into position by sleeve161 through threaded connection 156. The sleeve 161 serves as aradiation shield for the IHC 160, minimizing heat loss throughradiation, except at the emitting surface 157.

The indirectly heated cathode (IHC) 160 may be machined from a singletungsten cylinder. An exemplary IHC 160 may be about 0.375 inch thick,and is robust against both F etch and ion bombardment. As seen in FIG. 5c, the IHC 160 has the appearance of a thick circular disk joined to ahollow cylinder which has a bottom flange 159 which registers the IHC160 within its mounting part, cathode plate 163. Two or more circulargrooves 158 or saw cuts are machined into the cylinder to reduce theconduction of heat from the cathode emission surface 157 to the cathodeplate 163, reducing electron beam heating requirements. A similar groove153 is machined into the sleeve 161 to reduce heat transfer to thecathode plate 163.The sleeve 161 attaches to the cathode plate 163 viathreads in the plate 163 and the sleeve 161. The sleeve 161 serves twofunctions: it “locks down” IHC 158, and acts as a radiation shieldbetween the IHC 160 and its environment, reducing heating powerrequirements. Note that the IHC 160 and its sleeve 161 are enclosed bythe water-cooled cathode block 40 which is designed to absorb radiationto reduce overall source heating. Filament 162 is constructed ofapproximately 1 mm-thick tungsten wire twisted into a three-bend patternwhich provides fairly uniform emission current coverage onto the bottomof the IHC 160 disk. The filament 162 is attached to dual clamps 164 aand 164 b which conduct current through dual leads 165 a and 165 b to avacuum feedthrough and to a 60 A filament power supply. This powersupply, and hence the filament, is floated to a negative potentialrelative to the IHC by a high voltage power supply, so that electronemission current leaving the filament 162 is accelerated to the IHC 160,providing electron beam heating. This 2 A, 1 kV power supply provides upto 2 kW of electron beam heating power to bring the cathode surface 157to electron emission. In practice, 1 kW of electron beam heating issufficient (1.7 A at 600V, for example), but for IHC arc currents ofover several amperes, higher cathode temperature and hence higher poweris needed.

The IHC 160, sleeve 161, and filament 162 are preferably made oftungsten. The filament leads shown in FIG. 5 b are crimped onto thefilament 162, and are usefully made of molybdenum or tantalum, forexample. The cathode plate 163 can be made of graphite, stainless steel,molybdenum, or any high temperature, electrically conductive materialhaving good mechanical tensile strength. Since the cathode plate 163mounts directly to the cathode block, it is at cathode potential whenthe IHC 160 is energized.

FIGS. 5 d and 5 e illustrate the indirectly-heated cathode assembly 100mounted onto the water-cooled cathode block 40. A pair of water fittings41 a and 41 b are used to transport de-ionized water through a vacuuminterface. The water circulates through the cathode block 40 and canabsorb several kW of power, allowing the cathode block 40 to remain wellbelow 100° C. at all times. The IHC 160 is unipotential with the cathodeblock 40. As such, no insulation is required between the cathode 160 andcathode block 40, which forms the top and bottom boundary surfaces ofthe ionization volume 35. This results in a very reliable system, sincein prior art IHC sources, the IHC is up to 150V different from itsimmediate surroundings. This results, in turn, in quite common failuresprecipitated by the collection of debris between the IHC 160 and theionization volume surface through which it penetrates. Another benefitof the configuration is that it eliminates the common failure ofanticathode erosion since the top plate of cathode block 40 serves asthe anticathode since it is at cathode potential. The plasma column isbounded by the ionization volume 35 is defined by the bore through theionization volume liner 30 and the top and bottom plates of the cathodeblock 40. This defines a very stable volume to sustain the plasma columnduring arc discharge operation.

FIG. 5 f shows a detail of the magnetic yoke assembly 150 whichsurrounds the cathode block 40 and the ionization volume 35. Themagnetic yoke assembly 150 is constructed of magnetic steel and conductsmagnetic flux through an ionization volume or chamber assembly 90,producing a uniform axial magnetic field of about 120 Gauss, forexample, within the ionization volume 35. This magnetic yoke assembly150 is used to generate a magnetic field to confine the plasma generatedin the ionization volume 35 during an arc discharge mode of operation.During an electron impact mode of operation, the electron gun assembly110 is shielded from the magnetic field because of a magnetic shieldwhich is inserted between the yoke assembly 150 and the electron gun, asindicated in FIG. 7 below.]

FIGS. 6 a and 6 b illustrate the external electron gun assembly 110. Inparticular, Such electron gun assemblies are disclosed in detail in U.S.Pat. No. 6,686,595 as well as US Patent Application Publication No. US2004/0195973 A1, hereby incorporated by reference. FIG. 6 a is anisometric view of an exemplary emitter assembly 210 which forms a partof the external electron gun assembly 110. FIG. 6 b is an isometric viewof an electron gun assembly 110, shown with an electrostatic shieldassembly 250 removed. The electron gun assembly 110 includes a gun base240, which carries an emitter assembly 210, an anode 215, anelectrostatic shield assembly 250 and a magnetic shield 255.

Electrons emitted from a filament 200 in the emitter assembly 210 areextracted by the anode 215 and bent through 90 degrees by the magneticdipole 220, passing through an aperture 230 in the gun base 240. Theelectron beam is shielded from the magnetic fields within the ionizationvolume assembly 90, generated by the magnetic yoke 150, by a magneticshield 255. The anode 215, gun base 240, and the electrostatic shieldassembly 250 are all at anode potential, as high as, for example, 2 kVabove the potential of the ionization volume assembly 90, which is heldat the potential of the source block 120 during electron impactoperation. The filament voltage, for example, is several hundred voltsnegative; thus, the electron beam is decelerated between the gun base240 and the ionization volume 35, as described in detail, for example byHorsky in U.S. Pat. No. 6,686,595, hereby incorporated by reference.

FIG. 7 is a physical representation of the magnetic circuit associatedwith the electron gun assembly 110 and the magnetic yoke assembly 150.As shown, the magnetic circuit consists of the magnetic dipole 220, thegun magnetic shield 255, and the magnetic yoke assembly 150. Magneticdipole 220 is made of magnetic stainless steel, and produces a uniformtransverse magnetic field across the poles, bending the electron beamproduced by the electron gun emitter through approximately 90 degrees.Thus deflected, the electron beam passes through the aperture 230 ofFIG. 6, and into the ionization volume, where it is confined by thechamber magnetic field.

FIG. 8 is an isometric view of an exemplary dual hot vaporizer assembly301. The dual hot vaporizer assembly 301 includes dual vaporizer ovens300 a and 300 b, heater windings 310 a and 310 b, and a pair of vapornozzles 320 a and 320 b. Solid source material, such as As, P, Sb₂O₃, orInF₃, resides within the oven cavities, which are hollow steelcylinders. Sometimes the material is captured by a graphite cruciblewhich forms a liner between the material and cylinder, preventingcontamination of the oven walls. The oven heater windings 310 a and 310b carry up to 20 A of current at 48V DC, and can dissipate up to 1 kW ofheater power. They are brazed onto the ovens for good thermal contact.The nozzles 320 a and 320 b are usefully fabricated of molybdenum forgood temperature uniformity, but can be made of steel or other hightemperature, conductive materials. The nozzles are preferable ¼ inchtubing and no longer than two inches \long, to ensure good vaporconductance from oven to ionization volume. The temperature of the ovens300 a and 300 b is monitored by a pair of thermocouples 330 a and 330 b.The temperature of the heater windings 310 a and 310 b is monitored by apair of thermocouples 331 a and 331 b.

A mounting plate 340 is used to couple the dual hot vaporizer assembly301 to the source block 70. FIG. 9 a shows the source block 70 with thedual hot vaporizer assembly 301 removed while FIG. 9 b illustrates thesource block with the hot vaporizer assembly 301 being inserted.

FIG. 10 is a diagram which illustrates the typical voltages applied toeach element of the ion source when operating in electron-impactionization mode. All voltages are referenced to source potential Vs,which is positive with respect to ground. The mounting base or interfaceblock 50, the cathode block 40, and the source block 70 are held at Vs.The electron gun filament 200 is held at cathode potential Vc by itsrelated power supply (−1 kV<Vc<−100V), and the potential of the anode240 Va is positive (1 kV<Va<2 kV), so that the kinetic energy of theelectrons leaving the filament 200 and forming the electron beam 27 ise(Va−Vc). The ion extraction aperture plate 60 is biased to either apositive or negative voltage to improve the focusing of the extractedion beam (−350V<Vb<350V). The IHC assembly 100 is not energized duringan electron-impact ionization mode and is held at the potential Vsduring this mode.

FIG. 11 is similar to FIG. 10 but indicates the typical voltages appliedto each element of the ion source when operating in arc discharge mode.All voltages are referenced to source potential Vs which is positivewith respect to ground. The electron gun assembly 110 is not used, butthe cathode supply is connected to the IHC cathode 160 Vc (−100V<Vc<−0),which is unipotential with the cathode block 40. Since the electron gunassembly is not used in this mode, its filament 200 and anode 240 areheld at cathode voltage Vc. The IHC filament 162 is at up to 1 kV belowthe IHC 20 potential (−1 kV<Vf<0), and can provide up to 2 A, forexample, of electron beam heating current. The IHC 160 is up to 100V,for example, different from its immediate surroundings. FIGS. 12 a and12 b are logic flow diagrams of the sequence of steps required toestablish each operating mode in succession. Since the voltages of theion source components are different for the two modes of operation,there is a preferred sequence for moving between modes:

When switching from the electron impact mode 600 to the arc dischargemode 614, as illustrated in FIG. 12 a, initially, in step 602, theelectron gun assembly 110 is shut off. Next in step 604, the electrongun anode 215 is decoupled from its power supply. In step 606, theelectron gun anode 215 is set to cathode potential. This prevents anyfields from punching through the cathode block 40 at the upper plate ofthe cathode block 40, making this an effective anticathode. In step 608,the bias voltage applied to the ion extraction aperture plate 60 isinterrupted. The extraction aperture plate 60 bias is only needed incluster mode, and is not recommended in discharge mode, especially sincethe power supply may draw high currents due to the proximity of a denseplasma. Next in step 610 water flow into the cathode block 40 isinitiated by automatic sequencing of pneumatically actuated water flowvalves. The water flow valves are interlocked to the ion source controlsystem through a water flow sensor and relay switch so that the IHCcannot be energized unless flow has been established The cathode block40 must be water cooled during operation of the IHC assembly 100 toprevent undue heating of the source components, and to keep the magnets151 a, 151 b in the magnetic yoke 150 below their Curie temperature.Finally in step 612, an arc can by initiated by the introduction ofprocess gas into the ionization volume 35 and energizing the IHCassembly 100 as is known in the art.

When switching from the arc discharge 614 to the electron impact mode600, as illustrated in FIG. 12 b, initially in step, the IHC assembly100 is de-energized. Next in step 618, the electron gun anode 215 isconnected to its positive power supply. In step 620, the cathode block40 and the IHC assembly 100 are connected to the to the source voltage.In step 622, the bias voltage is set and connected to the ion extractionaperture plate 60. In step 624, water cooling of the cathode block 40 isterminated. Finally, in step 626, the electron gun assembly 110 isenergized to establish an electron beam. Also, vapor is introduced intothe ionization volume 35 to begin ionized cluster formation.

FIG. 13 shows the thermal interfaces between source block 70, theinterface block 50, the cathode block 40, and the ionization volumeliner 30. As further outlined in FIG. 4 a, thermal paths are definedbetween the cathode block 40, the ion extraction aperture 60, theinterface or mounting block 50, the ionization volume or source liner30, and the source block 70 through thermally conductive gaskets whichare in wetted contact to the surfaces of these components. Thus, theionization volume liner 30 can attain higher temperatures than thetemperature of source block 70, which is actively temperaturecontrolled. In addition, the water-cooled cathode block 40 has a thermalpath to reach the temperature of the mounting base 50 after watercooling is disabled.

FIG. 19 is a plot of mass-analyzed B₁₈H_(x) ⁺ beam current delivered toa Faraday cup positioned 2 meters from the ion source and downstreamfrom an analyzer magnet, and total ion current extracted from the ionsource. Shown are the extracted ion current, in mA, on the rightvertical axis, and the Faraday current (similar to on-wafer current) onthe left vertical axis. The currents are measured as a function ofB₁₈H₂₂ vapor flow into the ion source, measured as inlet pressure intothe ion source. The vapor was fed into this ion source through aproprietary closed-loop vapor flow controller which has been describedin detail elsewhere. The transmission through the extraction optics andbeam line of this implanter is about 25%, and begins to fall off at thehighest vapor flows, presumably due to charge exchange with the residualvapor.

FIG. 20 is a B₁₈H₂₂ mass spectrum collected from the ion source of thepresent invention, in electron-impact mode. The parent peak, B₁₈H_(x) ⁺,represents about 85% of the beam spectrum. The small peak at half theparent 210 amu mass is doubly ionized B₁₈H_(x) ⁺, or B₁₈H_(x) ⁺⁺.

FIG. 21 is a PH₃ mass spectrum collected from the ion source of thepresent invention, in arc discharge mode. Over 10 mA of ³¹P⁺ current andover 2 mA of doubly ionized phosphorus was delivered to the Faraday ofthe implanter at 20 kV extraction voltage. This performance iscomparable to many commercial Bernas-style ion sources used in ionimplantation.

FIG. 22 is an AsH₃ mass spectrum collected from the ion source of thepresent invention, in arc discharge mode. Over 10 mA of ⁷⁰As⁺ currentand about 0.5 mA of doubly ionized arsenic, as well as 0.5 mA of arsenicdimer was delivered to the Faraday of the implanter at 20 kV extractionvoltage. This performance is comparable to many commercial Bernas-styleion sources used in ion implantation.

FIG. 23 is a phosphorus spectrum showing the monomer P⁺, the dimer P₂ ⁺,the trimer P₃ ⁺, and the tetramer P₄ ⁺, produced in electron impactmode. The spectrum is unusual in that the monomer, dimer, and tetramerpeaks are all about the same height (about 0.9 mA), so that the tetrameryields the highest dose rate, or about 3.6 mA of effective phosphorusatom current. The spectrum was produced using elemental P vapors fromthe hot vaporizer of the dual mode source. The high cluster yield is dueto the fact that the P vapor preferentially produces P₄, and thisfragile cluster is preserved during the ionization process byelectron-impact ionization without exposing the vapors to intenseradiation or heat.

FIG. 24 is similar to FIG. 23, but collected with elemental As vaporsproduced by the hot vaporizer of the dual-mode source. The As spectrumshows the monomer ⁷⁰As⁺, the dimer As₂ ⁺, the trimer As₃ ⁺, and thetetramer As₄ ⁺. At 20 kV extraction, the equivalent of 4 mA of 5 keV As⁺is delivered to the Faraday.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than is specifically described above.

1. A universal ion source comprising an ionization :volume for ionizingsource gas or vapor; a cathode assembly for generating a plasma in saidionization volume in a first mode of operation; an electron gun forgenerating electrons in a second mode of operation, said electron gunjuxtaposed external to said ionization volume and configured to directelectrons into said ionization volume; a source of gas or vapor; andmeans for switching between said first mode of operation and said secondmode of operation.